Any inactive lifestyle is in danger of developing poor posture, which can lead to a host of other problems. However any activity will improve the flexibility of the two prime movers when it comes to posture and they are the abdominals and the back. Good posture comes from an equal balance between the two.
The strengthening of the back muscles specifically the rhomboids will directly improve posture. It is with movements like dead-lift, pull-ups or chin-ups and rows, whether bent-over with a barbell or seated with a cable. The various muscle groups in the back, with weight training will ensure both flexibility and strength when it comes to the back.
But as mentioned above developing a strong back is only fixing half the problem as the focal point of good posture is in the abdominal area along with strength in the lower back. The abdominals are the core stabilization of any movement that you do and they act as a lifting belt protecting your forward and back weight on your body from collapsing.
When training with weights one should always be very careful to balance the amount of time you spend training your chest as training your back. Any imbalance will result in either rolled shoulder, or hunchback with the head pulled forward known as kyphosis, which is a tell-tale sign of tight and overworked chest muscles.
Dead-lifting with good form and making sure that when you squat that you are always keeping to the strictest of form will always ensure that you get the flexibility required to avoid slipping into bad posture. With the barbell always traveling in a straight line up or down with gravity you can ensure that all the core stabilization muscles are used.
It is important to activate dormant muscles groups that will require full range of motion (ROM) in order to get full utilized in correct posture. This will ensure the flexibility and the strength to maintain good posture, which is so often ignored in the seated office jobs and passive lifestyles that we lead these days.
Posture comes together with attitude and good health, which can only be achieved by working all the muscles in the body to full range of motion. The inflexibility that results in bad posture is an indication of body both internally and externally and the state of a personís general health is often the first and sometimes the only sign a good doctor needs to make a diagnosis.